Heat transfer and defrosting characteristics of a horizontal single-row array of cooled tubes immersed in a gas-solid fluidized bed, have been studied experimentally. The fluidized bed produces gas-solid particle impinging jets to effectively remove frost layers on the tube surface. It has been verified that Frost-free running of the cooled tubes is possible under the conditions of inlet air temperature - 7 degrees C, tube-surface temperature - 17 degrees C and relative humidity 80%. The local contact frequency of particles onto the tube surface was measured by an optical sensing system and the defrosting mechanism of impinging solid particles was observed in detail with a visualization technique.